Personally I'd not pay more than £200-250 for that, although frankly I'd not touch it at all.
That Xeon came out at the start of 2016, don't get me wrong for someone looking to add cores or HT to an older setup to eke out some more performance they're brilliant, you can often get them for pennies. For a full build costing hundreds I'd nope out in that case, everything is going to be out of warranty and it's sitting on the aged side as is, I'd expect the price to reflect that.
For context you can get a second hand Xeon e5 2630 v4 for £18, and as a ten core it's not useful for gaming specifically as that very likely limits its overall potential for clockspeed, games are becoming more multithreaded over time but outside of extreme scenarios that would tank the system anyway, they're nowhere near the ten core + HT level.
It sounds like a dodgy ebay or facebook marketplace job. I'd also be concerned about the legitimacy of the GPU, which is the selling point, there's a lot of fakes out there.
